1. Field of the Invention
The present invention relates to what is called an auto focus technique for cameras which electronically records a subject image obtained by an image pickup element (imager) via a photographing lens.
2. Description of the Related Art
In general, an auto focus (AF) function of a camera provided with an image pickup element has hitherto been based on an AF system of focusing a photographing lens by varying the focus position of the photographing lens while determining the intensity of a contrast signal for an image obtained by the image pickup element at each focus position, that is, what is called an “imager AF system.” The imager AF requires a calculating process to be executed by obtaining image data at a plurality of focus positions immediately before photographing.
Thus, since the auto focusing based on the imager AF includes an operation of scanning the lens position of the photographing lens, a release time lag from depression of a release button till the start of driving of a shutter tends to be long. Techniques for reducing the release time lag have been proposed in Jpn. Pat. Appln. KOKAI Publication No. 2001-141985, Jpn. Pat. Appln. KOKAI Publication No. 2001-249267, and Jpn. Pat. Appln. KOKAI Publication No. 1999-023955; these techniques also use external light AF carried out by a distance-measuring device utilizing an optical system different from the photographing lens.
Another example of the imager AF in which the focus is detected for a plurality of points in a screen has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-255450.